This invention relates generally to a corrosivity sensor and a method for its manufacture, and more particularly to a corrosivity sensor which is light enough for aircraft applications and thin enough for use in laminated composites and under coatings, and which is manufactured using masking techniques.
The destructive effects of corrosion are well known and efforts to minimize those effects are ongoing. The non-nuclear naval aircraft-carrier environment is a particularly corrosive one, with sulfur from aircraft-carrier stack gases combining with sea spray to provide a hostile environment which undermines the structural integrity of naval aircraft. Sensors for early detection of the presence of corrosive elements beneath coatings and between the layers of laminated composites would prompt early aircraft maintenance and repair and prevent disaster as well as save time and money.
Galvanic cell-type corrosion probes are currently used to determine the corrosivity of a surrounding medium. One such probe is disclosed in U.S. Pat. No. 4,994,159 to Agarwala et al. This kind of probe has been used on naval aircraft carriers to monitor the corrosivity of the sea environment due to moisture and salt. In operation, a thin film of water from the environment in question, usually in the form of sea spray, contacts the surface of the probe, acting as an electrolyte to complete the galvanic cell formed by two dissimilar metals exposed at the surface. In other words, this film of water acts as a pathway for current flow between the surfaces of the dissimilar metals. The probe is made by embedding within a matrix of insulating material a plurality of plates of alternately anodic and cathodic material, and then exposing the edges of the plates to form the surface. The probe's large size and heavy weight limit its usefulness in applications where space and weight are critical issues, such as on aircraft, and, in particular, within the composite layers of aircraft or on aircraft surfaces to be painted. The large size of this probe also limits its sensitivity because it will only detect the presence of electrolytic films which are large enough in area to bridge the gap between the exposed metals. Additionally, since this probe is made by exposing the edges of flat plates of anodic and cathodic material, the surface pattern thus formed is limited to straight-line strips. The probe is therefore not sensitive to the presence of a film spreading along the insulating surface between strips and parallel thereto.
Masking techniques have been used to manufacture printed circuit boards, but have not been applied to the formation of corrosivity sensors.